This invention relates to a method of demodulating the position signal of a magnetic disk device. More particularly, the invention relates to a magnetic disk position signal demodulating method for reading servo burst signals, which have been recorded in a servo area by a head, demodulating a position signal, which indicates a positional deviation from a reference position on a track, using the servo burst signals that have been read, and outputting the demodulated signal.
Systems for recording the servo signals of a magnetic disk device are classified broadly into a sector servo system and a dedicated servo system. In the sector servo system, a data area for writing information recorded by a user and a servo area in which a servo signal used for head positioning has been written are provided on the same side of a magnetic disk. More specifically, according to the sector servo system, each track is divided into a plurality of units referred to as sectors, and part of the sector (usually the head of the sector) is provided with the servo area. This system is also referred to as an embedded servo system. In the sector servo system, format efficiency (the ratio of the data area to the total disk surface) does not depend upon the number of disks. Consequently, the sector servo system is often employed in recent devices having only a small number of disks. Since servo information is provided for each disk surface, another feature is that the system is not readily susceptible to the effects of head position offset caused by thermal expansion or the like.
The dedicated servo system differs from the sector servo system in that the servo area is provided on the side of the disk opposite that having the data area. Ordinarily, one surface among a plurality of magnetic disk surfaces is used as the servo area and the remaining disk surfaces are used as the data areas. The dedicated servo system often is used when a magnetic disk device employs a large number (ten or more) of disks.
In a magnetic disk device in accordance with the sector servo system, as shown in FIG. 10, signals (data signals and servo burst signals) that have been recorded on a storage medium (magnetic disk) 1 are reproduced by a magnetic head 2, and the reproduced signals are input to a read/write signal processing circuit 4 via a head ID 3 that controls head changeover or the like. The read/write signal processing circuit 4 subjects the reproduced signals to preprocessing necessary for signal demodulation and inputs the processed signal to a data demodulating circuit 5 and servo circuit 6. The data demodulating circuit 5 demodulates user data using a signal (data signal) that has entered from the read/write signal processing circuit 4. The servo circuit 6 demodulates a position signal, which is for controlling head positioning, using a signal (servo burst signal) that has entered from the read/write signal processing circuit 4, and inputs a control voltage command to a VCM (voice coil motor) driver 7 to control the VCM 8 so as to position the head 1 on a track. This is referred to as an "on-track" operation.
This magnetic disk device according to the sector servo system is such that a disk surface is divided into a plurality of sectors and each sector is divided into a servo area and a data area, as set forth above. FIG. 11 is a partially enlarged view of a magnetic disk and is useful in describing sector configuration. A number of tracks are formed on a disk surface from the outer periphery to the inner periphery thereof and each track is composed of a plurality of sectors. Each sector is divided into a servo area SVA and a data area DTA. The servo area SVA is composed of a sector mark SM, a track number TNO, and a servo burst signal SVP. As shown in FIG. 12, the servo burst signal SVP is formed by alternately recording, at regular intervals in the radial direction, burst patterns BP1, BP2 having a prescribed recording frequency. Tracks TR each have a width P. The boundary between the burst patterns BP1, BP2 in the radial direction is the on-track position.
In accordance with the servo burst signal, peak values P.sub.A, P.sub.B of head outputs read from the burst patterns BP1, BP2 are equal when the head 2 is at the center of the track (i.e., "on track"), as indicated at 2 in FIG. 13. As the head deviates from the center of the track, as indicated at 1, 3 in FIG. 13, a difference between the peak values increases. Accordingly, (P.sub.A -P.sub.B) can be adopted as a signal (a position signal) that conforms to the deviation from the center of a track. By arranging the tracking servo system (position servo system) so as to make the position signal (P.sub.A -P.sub.B) equal to zero, the head can be positioned at track center at all times, thus making it possible to perform read/write accurately.
Positioning is carried out in a magnetic disk device to move a head from the present track position to a target track position. In such control for positioning the head, first a command velocity conforming to the number of tracks to the target track is generated and velocity control is performed in such a manner that actual velocity will come into agreement with the command velocity. When the head arrives above the target track, control is changed over from velocity control to position control and the head is positionally controlled to attain the track center position so that the position signal will become zero.
The foregoing is for a case where the position signal is generated based upon the difference between the peak values PA and PB When the signals obtained by reproducing the burst patterns BP1, BP2 read by the head are full-wave rectified and subsequently integrated, the integrated values (see FIG. 14) become equal at the on-track position. By full-wave rectifying the respective burst patterns BP1, BP2 read by the head, obtaining the integrated values using an analog integrating circuit and adopting the difference between the integrated values as the position signal, effects similar to those of the peak-hold scheme are obtained.
With the conventional magnetic disk device described above, however, it is necessary to provide a special-purpose analog circuit when detecting the peak values of waveforms or the integrated values in order to demodulate the position signal. This results in circuitry of larger scale and is disadvantageous in terms of size and cost. Moreover, with a demodulating circuit constituted by an analog circuit, it is generally difficult to perform signal demodulation of a high-speed sampling.